High speed downlink packet access (HSDPA) is a feature introduced in Release 5 of the third generation partnership project (3GPP) specification. HSDPA achieves maximum spectral efficiency using three key concepts: adaptive modulation and coding (AMC), fast physical layer retransmissions by implementing hybrid automatic repeat request (HARQ), and fast Node B scheduling.
Handover is a process in which a wireless transmit/receive unit (WTRU) switches from one cell to another without service interruption. When a handover occurs, the WTRU needs to switch to a new serving HS-DSCH cell (target cell) and stop communicating with the old serving HS-DSCH cell (source cell). This procedure is also called serving HS-DSCH cell change.
A WTRU continuously monitors signal strength of neighboring cells and a serving HS-DSCH cell. When a signal strength measured on a common pilot channel (CPICH) of a neighboring cell exceeds that of the serving cell, (i.e., Event 1D), the WTRU sends a radio resource control (RRC) measurement report to a radio network controller (RNC) to report the change of the best cell. The measurement report triggered by Event 1D contains the measured value and the cell identity (ID).
Upon reception of the event 1D measurement report, the RNC makes a decision to perform a handover to a target cell. A serving radio network controller (SRNC) requests a controlling radio network controller (CRNC) to allocate HS-DSCH resources for the WTRU, (such as HS-DSCH radio network temporary identity (H-RNTI), high speed shared control channel (HS-SCCH) codes, HARQ resources, etc.), in the target cell via radio network subsystem application part (RNSAP) and Node B application part (NBAP) messages. Once the HS-DSCH resources are reserved the CRNC provides all the information to the SRNC which in turn sends an RRC message to the WTRU. The RRC message that may be used to indicate a serving HS-DSCH cell change may include physical channel reconfiguration, transport channel reconfiguration, radio bearer reconfiguration, active set update, or the like. The RRC message provides the WTRU with the radio access parameters required for the WTRU to start monitoring the target cell. In addition, the RRC message may provide an activation time at which the handover should take place.
The handover may be either synchronized or unsynchronized. In an unsynchronized handover, the network and the WTRU do not activate the resources and switch at the same time. The activation time for the WTRU is set to “now”. This reduces the delays associated with the handover procedure. However, it increases the probability of data loss.
In a synchronized handover, the network and the WTRU perform the change of resources simultaneously. However, the network has to set the activation time conservatively to account for any kind of delays, such as scheduling delay, retransmissions, configuration time, etc. Therefore, even though the synchronized handover minimizes data losses, it results in longer delays.
Conventionally, the RRC handover message is sent to the WTRU via the source Node B. The delay associated with the serving HS-DSCH cell change may cause the RRC handover message to fail, resulting in an unacceptable rate of dropped calls.
Several proposals have been made to optimize the serving HS-DSCH cell change procedure. In accordance with the proposals, a WTRU and a Node B may be pre-loaded, (i.e., pre-configured), with the HS-DSCH configuration information, i.e. target cell pre-configuration.
Target cell pre-configuration adds robustness to the serving HS-DSCH cell change procedure by allowing the network to send the serving HS-DSCH cell change command either over the source cell and/or the target cell using the HS-SCCH. The target cell pre-configuration is provided to the WTRU via an active set update procedure. More specifically, when a cell is added to the active set, (i.e., event 1A or 1C is triggered), the network sends an active set update message to the WTRU, which, in addition to the dedicated physical channel information, includes the HS-DSCH serving cell information of the new cell, (such as the H-RNTI, HS-DSCH, etc.).
When an event1D is triggered and the WTRU transmits a measurement report to request a serving cell change to the target cell, the WTRU starts to monitor the HS-SCCH of the target cell (using the target cell pre-configured information) in addition to the HS-SCCH set in the source cell. The network may then send an HS-SCCH in the target cell to trigger a serving cell change. Upon reception of the HS-SCCH on the target cell the WTRU executes the HS-DSCH serving cell change to this target cell.
In the conventional serving HS-DSCH cell change procedure, the WTRU performs a MAC-hs or MAC-ehs reset when an inter-Node B handover occurs, which is explicitly indicated to the WTRU from the network. The network signals to the WTRU if a MAC-hs/ehs reset is required via a MAC-hs/ehs reset indicator in the handover message. If the MAC-hs/ehs indicator is set, the WTRU performs a MAC-hs/ehs reset. Otherwise, a MAC-hs/ehs reset is not performed. With the introduction of the new changes in accordance with the above proposals, some problems may occur.
First, when Event 1D occurs (change of the best cell), the WTRU starts monitoring an HS-SCCH in the target cell while still monitoring an HS-SCCH(s) in the source cell. If an HS-SCCH order is received from the target cell confirming change of serving cell the WTRU has no method of determining whether a MAC-ehs or MAC-hs reset is required. This is due to the fact that the target HS-SCCH does not contain an explicit indicator to order the WTRU to perform a reset, as is done with the RRC handover command. A reset will be required if the network performed an inter-Node B handover or an intra-Node B handover whereby the MAC-hs or MAC-ehs context is reset on the network side. The WTRU has no method of determining that such a reset has been performed on the network side. A MAC-ehs or MAC-hs reset procedure comprises resetting the transmission sequence number (TSN) values to zero, flushing the hybrid automatic repeat request (HARQ) buffers, etc. If the WTRU does not perform a reset the WTRU will not be synchronized with the network, thus resulting in potential loss of data.
In addition, when a handover occurs the WTRU switches to the target cell to receive downlink data traffic. However, it is unclear what happens with the uplink traffic and how the WTRU is assigned the uplink resources of the target cell. According to the current 3GPP RRC specification, the serving HS-DSCH cell and the serving E-DCH cell have to be identical. Therefore, efficient methods to perform the handover in the uplink must also be defined.
Lastly, the network is not aware whether the WTRU supports the fast optimized handovers. Therefore, the network may start transmitting the handover message over the target cell, but the WTRU cannot receive it since the WTRU does not support such capability. This would cause the HS-DSCH cell change to fail and thus the WTRU would have to fall to CELL_FACH and initiate a cell reselection to the target cell.